The traditional system and method for measuring blood pressure in vertebrate subjects such as human beings involves controlling the flow of blood in an extremity to cause circulation to stop, and noting pressures corresponding to the presence of audible sounds that occur as the blood circulation is allowed to resume. The method commonly includes applying an inflatable cuff to an extremity of the subject, pressurizing the inflatable cuff with a working fluid such as air to apply sufficient pressure to cause cessation of circulation in the extremity, and listening with a device such as a stethoscope while allowing the pressure applied to the extremity to decrease so as to identify a systolic blood pressure corresponding to the beginning of blood flow, and a diastolic blood pressure corresponding to the unrestricted flow of blood in the extremity. Characteristic sounds are heard upon the initial resumption of blood flow, which sounds end when the blood flow becomes unrestricted. Pressure is commonly measured in millimeters of mercury (mm. Hg) using a gauge such as a sphygmomanometer. U.S. Pat. No. 5,704,362, issued Jan. 6, 1998 to Hersh et al., describes older systems that employ Korotkoff sounds, as well as systems that use the oscillometric technique. One common problem is the discomfort associated with the elevated pressure that is applied with such systems.
Newer systems involve electronic sensors for measuring pressure within the inflatable cuff, and electronic sensors to detect the oscillatory pressures that occur within the cuff as a consequence of the pulsatile displacement of blood within the extremity. For example, U.S. Pat. No. 3,903,872, issued on Sep. 9, 1975 to Link, describes a system and method for measuring the oscillatory blood pressure using a cuff applied to an arm of a patient, and sensing sphygmometric information electronically. U.S. Patent Application Publication No. 2003/0069507 A1, by Nishibyashi, describes a cuff applied to a leg of a patient, also using electronic sensing apparatus to obtain non-invasive blood pressure information. U.S. Pat. No. 4,860,759, issued Aug. 29, 1989 to Kahn et al., describes an apparatus that measures blood pressure signals using an inflatable cuff and associated sensor equipment, and that processes the blood pressure information in conjunction with other sensed information, such as a pulse signal recorded from a finger probe. U.S. Pat. No. 5,704,362 describes mathematical analysis methods for extracting blood pressure information by fitting curves to blood pressure signals. The entire disclosure of each of the above identified patents and published applications is incorporated herein by reference in its entirety.
A number of problems in addition to discomfort have been observed with systems involving electronic sensors. Some of the problems include the inability to make accurate measurements in the presence of motion in the subject extremity, and the inability to make accurate measurements on very young patients.
There is a need for blood pressure measurement systems and methods that provide accurate measurements for patients of all ages, that are fast and are comfortable in use, and that provide for inadvertent motion by the subject patient.